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Currently, the achievements called "Wings Of Liberty: Hard" and "Wings Of Liberty: Brutal" simply count the number of missions you have completed at each difficulty and give you an achievement for every 5 missions you have completed. So, you get a level of the achievement at 5, 10, 15, 20, and 25 missions completed. Right now, they both give you the final tier of the achievement once you complete 25 missions at that difficulty level.

Your quote is indicating that they are adding a final level above the "25 missions" reward. You also have to gain the "Liberty Completionist Hard" and "Liberty Completionist Brutal" achievements (which require you to have completed all 29 missions) before you get the final tier of the "Wings of Liberty" achievement for that difficulty.

For a long time there was a bug where, if you'd finished the campaign and replayed the missions on Hard/Brutal, then you wouldn't get credit until you'd done ALL of them, and then you'd get the 5,10,15,20,25 achievements all at once.
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SatanicpuppyMar 3 '11 at 17:06

The bug I experienced was such that I sometimes got a certain tier before I completed that number of missions and other times I would get that tier well after I completed the requirements. Either way you experienced the bug, this change and the related change (changing the progress bar to a mission list) are aimed at fixing that issue.
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ShaunMar 3 '11 at 17:10

my own experience was with a different bug -- replaying the same level would increment the "Complete X levels" counter, so I could replay the first 5 levels 5 times to get 'Wings of Liberty Brutal 25'.
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Raven Dreamer♦Mar 3 '11 at 18:25